Commercial aircraft frequently comprise an auxiliary power unit (also named “APU” for “Auxiliary Power Unit”) that is often situated in a tail cone of the aircraft and that is designed on the basis of a flow machine, for example a turbojet engine, that supplies the aircraft with compressed air and electrical energy. In such an arrangement the exhaust gas arising during operation exits through an exhaust gas outlet in the region of the tail cone into the surroundings of the aircraft so that to the largest extent possible under all conditions encountered both during a stopover of the aircraft on the ground and during flight operations none of the tail planes of the aircraft is in contact with the hot exhaust gas stream. Because of the frequent operation of the auxiliary power unit while the aircraft is situated on the ground, its noise may represent a noise nuisance to the airport and its environment. In order to reduce noise development on the ground, silencers are used that are coupled to the exhaust gas outlet of the auxiliary power unit and that correspondingly silence the exhaust gas noise.
From WO 2010/040837 A1 silencers for auxiliary power units of an aircraft are known whose basic design is particularly taken into account below.
A known silencer for an auxiliary power unit comprises a flow channel that connects an exhaust gas inlet to an exhaust gas outlet and that is enclosed by a housing. The volume between the housing and the flow channel is divided by partitions into several cells, wherein the majority of the partitions are arranged in an equidistant manner along the flow channel. Usually the flow channel comprises a metallic felt-like material that withstands the temperature and corrosive nature of the exhaust gas while at the same time being porous. This makes it possible not only to feed the exhaust gas through the flow channel along its extension in the housing, but also to bring about a changing flow across the aforesaid through the wall material of the flow channel into the individual cells so that during the passage through the wall material of the flow channel acoustic energy is converted to heat as a result of friction, and in this manner the exhaust noise is reduced.
In the design of a corresponding silencer, in particular in aircraft, the available installation space is severely limited so that in order to increase the extent of sound attenuation the outer dimensions of the silencer can practically not be enlarged but should always be kept constant or at least on the lowest-possible level. With the existing design methods of silencers for auxiliary power units of an aircraft, improvements to the extent of sound attenuation cannot be expected without further ado without the dimensions of the silencer being enlarged.